Quadrupole ion traps are devices in which ions are introduced into or formed and contained within a trapping volume formed by a plurality of electrode or rod structures by means of substantially quadrupolar electrostatic potentials generated by applying RF voltages, DC voltages or a combination thereof to the rods. To form a substantially quadrupole potential, the rod shapes are typically hyperbolic.
A two-dimensional or linear ion trap typically includes two pairs of electrodes or rods, which contain ions by utilizing an RF quadrupole trapping potential in two dimensions, while a non-quadrupole DC trapping field is used in the third dimension. Simple plate lenses at the ends of a quadrupolar structure can provide the DC trapping field.
When using a mass selective instability scan in a linear ion trap, the ions are most efficiently ejected from the trap in a radial direction. Some researchers have ejected ions between two of the quadrupole rods. However, due to high field gradients loss of ions is substantial. To increase the efficiency ions are ejected through a rod by introducing an aperture in the rod. For the linear ion trap, one manner in which an aperture can be introduced is along the length of the rod. When an aperture (or apertures) is cut into one or more of the linear ion trap electrodes to allow ions to be ejected from the device, the electric potentials are degraded from the theoretical quadrupole potential and therefore the presence of this aperture can impact several important performance factors. Consequently, the characteristics of this aperture are significant.
The introduction of an aperture into a linear ion trap not only may degrade the theoretical quadrupole potential, but may also contribute to the degradation of the structural integrity of the rods themselves, thus leading to mechanical deviations in the axial direction and ultimately affecting the performance characteristics such as the resolution attainable by such an ion trap mass spectrometer.
The performance of such a two-dimensional ion trap is more susceptible to mechanical errors than a three-dimensional ion trap. In a three-dimensional ion trap, all of the ions occupy a spherical or ellipsoidal space at the center of the ion trap, typically an ion cloud of approximately 1 mm in diameter. The ions in a two-dimensional ion trap, however, are spread out along a substantial fraction of the entire length of the ion trap in the axial direction which can be several centimeters or more. Therefore, geometric imperfections, misalignment of the rods, or the mis-shaping of the rods can contribute substantially to the performance of the two-dimensional ion trap. For example, if the quadrupole rods are not parallel along the substantial length of the rods, then ions at different axial positions within the ion trap experience a slightly different field strength. This variation in field strength experienced will in turn cause the ejection time of the ions during mass analysis to be dependent on the axial position. The net result for an ion cloud of the same m/z is increased overall peak widths and degraded resolution.
In addition to mechanical errors causing axial field inhomogeneity, the fringe fields caused by the end of the electrodes as well as the ends of any slots cut into the rods can also cause significant deviation in the strength of the radial quadrupole field along the length of the device. Ideally to keep the electric fields uniform, the ejection aperture would extend along the entire length of the rod, but this presents numerous construction challenges. To avoid these, ejection slots are typically located only along some fraction of the central region (for example 60%) of the total ion trap length. This however, would lead to a variation in the radial quadrupolar potential near the ends of the slots in addition the effects at the ends of the rods. Ions which reside in these areas would would be ejected at different times than ions residing more in the center of the device and therefore would result in a reduction in mass resolution.
One approach to produce a homogenous electric field is shown in FIG. 1 which depicts a two-dimensional quadrupole structure 100 having hyperbolic rods 105, 110, 115 and 120, each rod 105, 110, 115, 120 cut into three axial sections, Front section (a), Center Section (b) and Back Section (c). These three sections, each with a discreet DC level, allow containment of the ions along the axis in the Center Section (b) of the ion trap. More details on this structure can be found in U.S. Pat. No. 5,420,425. The use of a linear ion trap in which the rods are segmented provides one way in which to minimize the axial variation of the electric fields towards the ends of the rods and therefore to minimize its affect on the performance. This architecture creates a radial trapping potential which is very homogenous in the region where the ions are contained within the central section of the trap.
In the two-dimensional linear ion trap configuration discussed in the U.S. Pat. No. 5,420,425 patent, 12 V applied to the front and beck sections creates an axial trapping potential which is able to confine the ions to the central 25 mm (+/−12.5 mm from center) of the quadrupole structure 100 (if the axial energies remain below 1 eV). The aperture 125 has a length of approximately 29 mm and so allows efficient ion ejection —while maintaining a high level of axial homogeneity of the radial quadrupolar potential in the region containing the entire ion cloud. This can be seen in FIG. 2, trace 205 which shows the axial potential as a function of axial position.
The voltages necessary to operate such a two-dimensional, three-sectioned quadrupole structure 100 equates to nine separate combinations of voltages applied to twelve electrodes (including the DC voltages applied to the separate sections of each road to produce an axial trapping field, the RF voltage applied to the rod pairs to produce the radial trapping field, and the AC voltage applied across one pair of rods for isolation, activation, and ejection of ions). This requires the construction of a considerably elaborate RF/AC/DC system.
A simpler design for a linear ion trap uses single rod sections 305 with axial trapping provided solely by DC voltages applied to the end lenses 310, as illustrated in FIG. 3. This reduces the number of discreet voltages from nine to three, significantly reducing the complexity of the electronics system. A significant disadvantage of this design is that the axial trapping fields do not penetrate well into the interior of the ion trap, allowing ions to travel further from the center of the trap. This can be seen in FIG. 2, trace 210, which illustrates that when 200 V is applied to the end lenses, ions with 1 eV of axial energy expand to cover approximately 40 mm (+/−20 mm from center). This allows the ions to experience more axial field inhomogeneities due to the fringe fields at the end of the rods and the finite length of the ejection aperture.